1. Field of the Invention
The present invention relates to a semiconductor device including a fuse, and more particularly, to a method for testing whether or not the fuse is disconnected.
2. Description of the Related Art
Fuses may be incorporated into semiconductor devices. For example, in a semiconductor memory device incorporating normal memory cells and redundancy memory cells, not only a normal decoder for selecting the normal memory cells but also a redundancy decoder for selecting the redundancy memory cells are provided. That is, if a defective memory cell is found in the normal memory cells, an address of such a defective memory cell is written into the redundancy decoder. As a result, when this address is received by the redundancy decoder, the redundancy decoder deactivates the normal decoder, and in its place, selects the redundancy memory cell, to thereby replace the defective memory cell with the redundancy memory cell. Thus, the problem of the defective memory cell is alleviated.
The redundancy decoder Is usually comprised of a multi-bit fuse-type programmable read-only memory (PROM). Therefore, writing operation of the above-mentioned address into the redundancy decoder is carried out by laser trimming of fuses or by supplying a blowing current thereto (see: JP-6-140510-A).
After a fuse is disconnected by the above-mentioned writing operation, it is necessary to ensure that the fuse is completely disconnected.
In a prior art method for testing whether or not a fuse is disconnected, it is determined whether or not a voltage at one node of the fuse upon application of a predetermined voltage between the nodes thereof is larger (or smaller) than a threshold voltage. In other words, it is determined whether or not the resistance value of the fuse is larger than a threshold resistance value (see: JP-6-140510-A).